The ranges of the instruments in HALion Symphonic Orchestra were set some 15 years or so ago when that library was recorded, engineered and programmed. There’s zero chance of the HALion sound design team returning to HSO now to change these patches, I’m afraid. Either use the GM cello sound, which covers the whole range (but which doesn’t support techniques like pizzicato etc.), or use a different library that provides the range you’re looking for.